1. Field of the Invention
The present invention relates to rotary joints for fluids, and more specifically to rotary joints that allow fluidsxe2x80x94such as polishing solution for polishing the surface of silicon wafer by the chemical mechanical polishing (CMP) techniquexe2x80x94to flow through the components that rotate relative to one another.
2. Description of the Prior Art
An apparatus for polishing the surface of silicon wafers by CMP, to which this invention relates, has been developed in recent years. The apparatus, as shown in FIGS. 10 and 11, comprises: a rotary table 102 that rotates horizontally; a pad shaft support block 103 which moves horizontally back and forward and up and down; a polishing pad shaft 104 which, held by the pad shaft support block 103, is forced to rotate; a slurry fluid feeding and discharging passage 105 formed on the non-rotary side in the pad shaft support block 103; a feeding and discharging mechanism 107 connected to the slurry fluid feeding and discharging passage 105 for a polishing solution 106, for example, a KOH-contained silica slurry mixed in isopropyl alcohol; a slurry fluid feeding and discharging passage 108 on the rotary side which runs through the polishing pad shaft 104 and opens at the central portion of a pad head 104a; and a rotary joint 101 which, installed between the pad shaft support block 103 and the polishing pad shaft 104, connects the two slurry fluid feeding and discharging passages 105 and 108 in such a way that the two passages 105 and 108 are relatively rotatable when communicating with each other.
By that surface polishing apparatus, the silicon wafer 109 is polished in this way: first, the silicon wafer 109 is held on the rotary table 102, the surface 109a up, and the polishing pad shaft 104 is moved down until the pad head 104a comes into contact with the wafer surface 109a. Then, the polishing solution 106 is jetted out to between the pad head 104a and the wafer 109 by means of positive pressure action (jetting operation of the polishing solution pump) of the feeding and discharging mechanism 107. And the polishing pad shaft 104 is rotated and moved back and forward horizontally to polish the wafer surface 109a. After polishing is over, the feeding and discharging mechanism 107 is switched to negative pressure action (sucking operation of the polishing solution pump) to suck and remove the residues of the polishing solution 106 in the slurry fluid feeding and discharging passages 105 and 108. That is, care is taken so that the residues of the polishing solution 106 in the slurry fluid feeding and discharging passages 105 and 108 may not drop on the polished surface of the wafer, and that is effected by switching the slurry fluid feeding and discharging passages 105, 108 from the positive pressure mode to the negative pressure or dry mode.
The rotary joint 101 mounted in that surface polishing apparatus is designed as in the following. A first joint body, which is to be mounted on the pad shaft support block 103, is connected to a second joint body, which is to be fixed on the polishing pad shaft 104 such that the firs joint body and the second joint body may rotate relative to one another. Within the first joint body is formed a first fluid passage section which is connected to the slurry fluid feeding and discharging passage 105 on the non-rotary side. In the second joint body on the rotary side is formed a second fluid passage section that is connected to the slurry fluid feeding and discharging passage 108. A space formed between the two slurry fluid passage sections is sealed with a sealing member placed between the relatively rotating opposed faces of the first joint body and the second joint body. An example of such a sealing member is sealing faces formed on the opposing parts of the relatively rotating first and second joint bodies that are brought into contact with and pressed against each other. Another example to seal the relatively rotating parts is elastic seal materials such as O-ring.
The rotary joint 101 of such a design presents the following problems. That is, the polishing solution 106 is a slurry fluid containing abrasive grains. Those abrasive grains tend to intrude and be deposited between the sealing faces, making it difficult to keep the sealing function in a good shape for a long period. Also, the sealing faces will be worn in contact with the polishing solution 106, losing sealing function in a short period. Another problem is that wear particles from the seal faces and ingredients dissolving out of the elastic seal will get mixed in polishing solution 106, hampering the polishing of wafer surface 109a. The intrusion and deposition of such abrasive grains and the wearing of the sealing faces occur more evidently in particular by switching the slurry fluid feeding and discharging passages 105, 108 from positive pressure to negative pressure or dry mode. Especially in the dry mode, in addition, seizure will be inflicted on the sealing faces because of frictional heat. If the intrusion and deposition of abrasive grains and the wearing of the sealing faces affect the seal performance, polishing solution 106 can leak out of the sealing faces, causing such problems as staining wafer surface 109a and creeping into the bearings between the first and second joint bodies and hindering the polishing pad shaft 104 from rotating smoothly. And good polishing could hardly be hoped for of such a rotary joint 101.
Those problems are encountered with the prior art rotary joint 101 not only in the aforesaid surface polishing apparatus but in a rotary equipment in which a slurry fluid like polishing solution or a corrosive fluid must flow between component parts rotating at a rate higher than a certain level. Such being the case, it has been keenly desired that a solution to the problems should be found, but the fact is that no rotary joint for fluids has been developed which exhibits a stabilized sealing performance for a long time.
Accordingly, it is an object of the present invention to provide a rotary joint which permits smooth flow of a flurry fluid such as a polishing solution or a corrosive fluid through relatively rotating component parts without leakage and which makes a surface polishing apparatus or other equipment properly perform the functions as mentioned earlier.
It is another object of the present invention to provide a rotary joint that always exhibits a good and stable sealing performance irrespective of the sealing conditions such as the pressure and properties of fluid, thus perfectly preventing the contamination of fluids and the environment and which is suitable for use in a variety of equipment where a high degree of cleanness is required.
It is still another object of the present invention to provide a rotary joint which permits simultaneously smooth flow between the two joint bodies of a slurry fluid like polishing fluid and one or more kinds of liquids or gases, thus opening up a wide range of applications.
It is yet another object of the present invention to provide a rotary joint that can be reduced in size to a maximum extent.
Those objects are attained by a rotary joint constructed according to the present invention.
The rotary joint of the present invention comprises: a first joint body; a second joint body connected to the first joint body such that the second joint body is allowed to rotate in relation the first joint body; and a prime seal unit installed between opposed end portions of the two joint bodies, the opposed end portions arranged in the direction of the axis of rotation; and a continuous line of prime fluid passage which runs through the two joint bodies. The prime seal unit is a mechanical seal comprising: a stationary seal ring fixed concentrically on one of the opposed end portions of the two joint bodies with the axis of rotation as its center; a movable seal ring held on the other of the opposed end portions, concentric with the stationary seal ring and movable in the axial direction; a rotation stopper provided in the outer circumferential portion of the movable seal ring for preventing the movable seal ring from relative rotation while allowing the movable seal ring to move in the axial direction; and a thrusting mechanism that urges the movable seal ring against the stationary seal ring. And the seal unit is so built as to work as a seal between the outer and inner circumferential regions by the sliding contact between the relatively rotating two ring seals. The prime fluid passage is formed out of the inner circumferential or inside region of the two seal rings, a first prime fluid passage section which passes through the first joint body and opens at the inside region, and a second prime fluid passage section which passes through the second joint body and opens into the inside region.
In a preferred embodiment, one of the two opposed end faces of the two seal rings in the aforesaid seal unit is tapered or sharpened. In other words, one of the opposed seal end faces of the seal rings is in a circular form with a small width in the radial direction. The width in the radial direction of the circular face is set at preferably 0.1 to 0.8 mm, more preferably 0.4 to 0.7 mm. It is also desirable that the inner and outer circumferential faces forming the tapered or sharpened seal are conical in sectional shape and are at an identical angle of 105 to 150xc2x0 C. relative to the seal end face.
It is also desired that the prime seal unit is formed as a mechanical seal with 0xe2x89xa6Kxe2x89xa60.6 wherein K is the balance ratio.
The seal rings in the prime seal unit are made of silicon carbide, aluminum oxide, fluororesin or PEEK (polyether ether ketone). It desirable that at least the seal end faces of the seal rings should be made of silicon carbide. Preferred grades of silicon carbide for the purpose are not higher than 200 ppm in total metal content.
It is desirable that the stationary seal ring is made in a cylindrical form and fitted over the end of the joint body or fitted into a recess formed in the end portion of the joint body.
In case it is required that the prime fluids flowing in the main fluid passage not be contaminated with metals in the rotary joint, it is desired that the parts of the prime fluid passage which come into contact with the fluids should be made of a material that does not release metal components when coming into contact therewith.
The material that does not release metal components when into contact with the fluid means a material that does not give off metal ions when coming into contact with the fluid flowing through the prime fluid passage or which, in case the fluid contains solid ingredients such as abrasive grains, does not produce metal particles when coming into contact with the solid ingredients. Among such materials are generally plastics and silicon carbide. The ways of forming with such a material the parts of the fluid passage coming into contact with the fluid include the following two examples: one in which only the parts coming in direct contact with the fluid is made of those materials as by coating; and the other case where the component parts of the two joint bodies in which fluid passages are formed, or all the component parts of the two joint bodies and the seal rings, are made of those materials. It is desired that at least the inside walls of the respective prime fluid passage sections (including the component parts of the two joint bodies or the whole of the two joint bodies) are made of a plastic material inert in or resistant to the flowing fluid, for example. The inert or resistant plastic material is selected on the basis of the properties of the fluid. If, for example, the fluid contains solid ingredients such as abrasive grains, a plastic material to be selected should be free from wearing and releasing particles when coming into contact with the solid ingredients. If the fluid is hot in temperature, the plastic material to be selected should be thermo-resistant. If the fluid is corrosive, the plastic material to be selected should be resistant to corrosion. Concrete examples are engineering plastics such as PEEK, PES (polyethersulfone), and PC (polycarbonate) which are free from wearing and releasing particles when coming into contact with the solid ingredients such as abrasive grains and excellent in work dimensional stability and heat resistance and corrosion-resistant plastics such as PTFE (polytetrafluoroethylene plastic), PFA (tetrafluoroethylene perfluoroalkoxy vinyl ether copolymer), and FEP (fluorinated ethylene propylene copolymer plastics).
In a preferred embodiment, there is provided a cooling water space where the cooling water is supplied and circulated to cool the contact area of the two seal rings. This space is a region on the outer circumferential side of the two seal rings, formed between the outer circumferential surface of the second joint body and the inner circumferential surface of the first joint body and sealed with a seal unit provided on the circumferential surfaces and the prime seal unit. In this arrangement, it is desirable that the first joint body is provided with an inlet port and an outlet port which open into the cooling water space so that the cooling water may be circulated.
Furthermore, in case a fluid has to flow through a passage other than the prime fluid passage, a continuous line of auxiliary fluid passage is formed from two auxiliary fluid passage sections and a connecting region. The connecting region is a space between the outer circumferential surface of the second joint body and the inner circumferential surface of the first joint body which concentrically encircles the second joint body, and is sealed with a couple of circumferential side seal units placed therebetween and arranged in the direction of axis of rotation. The first and second auxiliary fluid passage sections open at the connecting region and are formed in such a way as not to cross the prime fluid passage sections.
A preferred circumferential seal unit is a mechanical seal comprising a stationary seal ring that is fixed to the inner circumferential surface of the first joint body and a rotary seal ring which is held on the outer circumferential surface of the second joint bodyxe2x80x94concentrically with, and opposite to the stationary seal ring, movable in the axial direction and urged against the stationary seal ring.
In such an arrangement, it is desirable that the inner circumferential surface of the stationary seal ring in at least one of the circumferential side seal units serves as a ring-formed bearing fitted over the outer circumferential surface of the second joint body, while allowing the second joint body to remain rotatable. In the circumferential seal units, a plate spring elastically changeable in form in the axial direction is preferred as thrusting mechanism to urge the rotary seal ring against the stationary seal ring. The seal rings in the circumferential side seal unit is made of silicon carbide, aluminum oxide, fluororesin, PEEK, or carbon. It is desired that one of the two seal rings in the circumferential side seal unit is made of silicon carbide and the other made of carbon.
In the respective joint bodies, it is also desired that at least the inside wall of the auxiliary fluid passage sections is made of a plastic material inert in or resistant to the fluid that flows through the auxiliary fluid passage sections. Depending on the properties of the flowing fluid, a plastic material is selected from among such engineering plastics as PEEK, PES, and PC, and corrosion-resistant plastics such as PTFE, PFA, and FEP.
The contact areas between the seal rings in the prime seal unit and in the circumferential side seal unit and the joint bodies are secondarily sealed by an O-ring. A preferred O-ring is made of fluororubber or fluororesin.